Supercapacitor backup power supply with bi-directional power flow

ABSTRACT

A system for providing backup power supply to a device is provided. The system includes a supercapacitor and a single circuit for charging and discharging of a supercapacitor. The single circuit operates with an inductor to provide for charging and discharging of the supercapacitor.

FIELD OF INVENTION

The present invention relates to power supply technology and moreparticularly to a supercapacitor based system for backup power supply.

BACKGROUND OF THE INVENTION

Many digital systems require a backup power supply for instances wheremain power becomes unavailable. Typically this has been done usingbatteries, but with the development of very high value capacitors(supercapacitors) it is quite often preferable to replace a battery witha capacitor. This is done mainly for service reasons: supercapacitorscan endure more charge/discharge cycles than rechargeable batteries, andhave a longer useable life than batteries leading to reduced serviceneeds for a given product requiring a backup mechanism.

Known backup power mechanisms using supercapacitors for energy storagecomprise two separate circuits: a circuit to charge the supercapacitorwhen a main power supply is available, and a switching power supplyrunning off the supercapacitor when the main power supply isunavailable.

A simple example of a backup power mechanism with separate charge anddischarge circuits is presented in FIG. 1. When the main power supply(not shown) is available, Vcc is generated by this power supply. Duringthis time, a switch 102 is closed allowing a supercapacitor 104 tocharge via a current source 103. The current source 103 may include aresistor, active current source, switching supply or other mechanism. Aswitch 106 is open during charging. The switch 102 is modulated tomaintain a fixed (maximum) voltage on the supercapacitor 104. This willgenerally be performed by a control mechanism (not shown).

When the main power source is lost, the switch 102 is opened and theswitch 106 is modulated to transfer energy from the supercapacitor 104to Vcc via an inductor 108 and a diode 110. Output filtering isperformed by output capacitors of the main power supply (not shown).Thus there are separate charge and discharge circuits. This use ofseparate circuits for charge and discharge requires additional partcount thereby adding cost, Printed Circuit Board (PCB) layout area andweight.

A higher efficiency can be achieved when the diode 110 has a switchacross it to form a synchronous rectifier. A circuit having thisadditional component is shown in FIG. 2. A switch 202 is connected inparallel with the diode 110. However, the circuit of FIG. 2 has aseparate charge and discharge circuit.

There are supercapacitor charging schemes of the art that only providefor simple charging mechanisms where the supercapacitor is placeddirectly across the voltage allowing a very large current at the startof charging.

There is therefore a need to provide a supercapacitor based backup powersystem that minimizes part count, provides efficient output voltagegeneration and provides controlled (the instantaneous currentrequirements of the voltage source are limited) and power-efficientcharging of the supercapacitor.

SUMMARY OF THE INVENTION

The present invention generally relates to the charging and dischargingof a supercapacitor that is used power supply backup situations.

It is an object of the invention to obviate or mitigate at least one ofthe drawbacks of prior art circuits used for the charging anddischarging of a supercapacitor.

In accordance with an aspect of the invention there is provided a systemfor backup power supply. The system includes a supercapacitor, and asingle circuit for charging and discharging of the supercapacitor. Thesingle circuit includes a path having an inductor for operating incharging mode for the charging and in backup mode for the discharging.

In accordance with another aspect of the invention, there is provided asystem for backup power supply. The system includes a supercapacitor, aninductor, a single circuit operating with the inductor to provide forcharging and discharging of the supercapacitor, and a controller formonitoring and controlling the single circuit.

This summary of the invention does not necessarily describe all featuresof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the invention will become more apparent fromthe following description in which reference is made to the appendeddrawings wherein:

FIG. 1 is a schematic diagram illustrating a supercapacitor based backuppower supply circuit of the prior art;

FIG. 2 is a schematic diagram illustrating another supercapacitor basedbackup power supply circuit;

FIG. 3 is a schematic diagram illustrating a supercapacitor based backuppower supply circuit in accordance with an embodiment of the presentinvention;

FIG. 4 is a schematic diagram illustrating a supercapacitor based backuppower supply circuit in accordance with another embodiment of thepresent invention;

FIG. 5 is a schematic diagram illustrating a supercapacitor based backuppower supply circuit in accordance with a further embodiment of thepresent invention;

FIG. 6 is a schematic diagram illustrating an example of a controlcircuit in accordance with an embodiment of the present invention; and

FIG. 7 is a schematic diagram illustrating a supercapacitor based backuppower supply circuit in accordance with a further embodiment of thepresent invention.

DETAILED DESCRIPTION

Embodiments of the present invention provide a backup power supply whichis implemented by a single charge-discharge circuit for asupercapacitor. The circuit may have a reduced part count compared tocircuits with separate charge and discharge circuitry. In thedescription below, the term “connect(ed)” may be used to indicate thattwo or more elements are directly or indirectly in contact with eachother.

FIG. 3 illustrates a supercapacitor based backup power supply circuit inaccordance with an embodiment of the present invention. The backup powersupply circuit 300 of FIG. 3 includes switches 302 and 304, diodes 305and 306, an inductor 308, and a supercapacitor 310. The switch 302 isconnected in parallel with the diode 305. The switch 304 is connected inparallel with the diode 306. The inductor 308 and the supercapacitor 310may be same or similar to the inductor 108 and the supercapacitor 104 ofFIG. 2, respectively. It is noted that FIG. 3 is conceptual in the sensethat further circuitry around that presented in FIG. 3 may be included.

The diode 306 acts as a so-called free-wheeling diode. The combinationof the switch 302, the inductor 308 and the diode 306 provides aswitching power supply or so-called buck converter that can be used tocharge the supercapacitor 310. As this circuit 300 can be used forcharging, a current source and its controlling switch (103 and 102 ofFIG. 2) become redundant. Therefore the circuit 300 does not use thecurrent source 103 and its switch 102 of FIG. 2. The circuit 300provides for both charging and discharging of the supercapacitor 310without a current source and its switch. In the circuit 300, magneticelement, i.e., inductor 308, operates in a bi-directional mode.

The circuit 300 is in charging mode when Vcc is generated by a mainpower supply (not shown). In charging mode, the switch 302 is modulatedto charge the supercapacitor 310 to a desired level, i.e., power flowsfrom Vcc to the supercapacitor 310. In charging mode, the switch 304 isgenerally left open at this time. It may however be closed during thefreewheeling time of the diode 306 for improved efficiency. In this casethe switch 304 behaves as a synchronous rectifier.

The circuit 300 is in backup (discharging) mode when the main powersource that generates Vcc is detected as missing. In backup mode, theswitch 304 is modulated such that power flows from the supercapacitor310 to Vcc. In backup mode, the switch 302 is used as a synchronousrectifier and is closed during the fly-back time of the inductor 308.

In an embodiment, a controller is provided to the circuit 300 to monitorthe main power source, supercapacitor voltage, output voltage (Vcc),inductor current (if current mode control is to be implemented), orcombinations thereof, and then control the operation of thecharge-discharge circuit based on the monitored value(s) (e.g., FIGS.4-6).

In one example, the controller monitors the main power source andenables the supercapacitor charging mechanism (charging mode) when themain power source is available. In charging mode, the controllermonitors the voltage across the supercapacitor 310 and operates theswitches 302 and 304 in conjunction with the inductor 308 such that abuck converter (with synchronous rectifier) is formed. In this caseenergy flows from Vcc to the supercapacitor.

When the main power source is lost, the controller then switches to thebackup mode. In backup mode, the controller monitors the voltage Vcc andruns the switches 302 and 304 in conjunction with the inductor 308 suchthat a boost converter (with synchronous rectifier) is formed. In thiscase energy flows from the supercapacitor to Vcc.

In either charging or backup mode, the controller may implement thecurrent mode control. The current mode control uses an inner controlloop to limit the peak or average current in the inductor 308, whichresults in the apparent removal of the pole associated with the inductor308 when compared to a voltage mode controlled switching mode powersupply. This resulting reduced order transfer function allows for betterdynamic response of the power supply, and may make the compensation ofthe power supply easier. For such control the controller includes amechanism to monitor the current of the inductor 308 in the currentcontrol mode. The inherent control of inductor current from the currentmode control works well with the concept of charging the capacitor at afixed rate. The circuit 300 may employ a voltage mode control forcontrolling the output voltage.

The circuit 300 is appropriate for the configuration where the supplyvoltage, Vcc, is greater than or equal to the maximum allowablecapacitor voltage. However, it is well understood by one skilled in theart that the circuit 300 can be restructured such that a Vcc lower thanthe maximum supercapacitor voltage can be supported. Thus a boostcircuit to charge the supercapacitor, and a buck circuit to supply Vccin backup is provided, i.e., a bi-directional power flow through onecommon mechanism.

FIG. 4 illustrates a supercapacitor based backup power supply circuit inaccordance with a further embodiment of the present invention. Thesupercapacitor based backup power supply circuit 401 of FIG. 4 issimilar to the circuit 300 of FIG. 3. The circuit 401 includes switches402 and 404, diodes 405 and 406, inductor 408, and supercapacitor 410.The diodes 405 and 406 correspond to the diodes 305 and 306 of FIG. 3.The inductor 408 may be same or similar to the inductor 308 of FIG. 3.The supercapacitor 410 may be same or similar to the supercapacitor 310of FIG. 3. The switches 402 and 404 correspond to the switches 302 and304 of FIG. 3. However, in this embodiment the switch 402 and 404 areMetal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs). In thedescription, the terms “switch 402 (404)” and “MOSFET 402 (404)” may beused interchangeably.

In one example, the diodes 405 and 406 may be intrinsic diodes of theMOSFETs 402 and 404, respectively. In another example, the diodes 405and 406 may be external schottky diodes connected in parallel with theintrinsic diodes of the MOSFETs 402 and 404, respectively. The schottkydiode may provide a current path during the time it takes for thecorresponding MOSFET to fully turn on. The schottky diode has a lowerforward voltage than the parallel diode that is intrinsic to theconstruction of the MOSFET, which is efficient for use in powerrectification applications.

The diodes 405 and 406 and the switches 402 and 404 and an inductorcurrent sensing mechanism may be integrated into an IC package(integrated circuit) with a controller 412. The controller 412 may beimplemented in any appropriate fashion. The inductor 408 and thesupercapacitor 410 may be outside of any integrated circuit.

In order for the controller 412 to provide the required functionality itreceives as input and is responsive to various signals. Such signalsaccording to an embodiment of the invention are presented in FIG. 4. A“˜MODE” control signal 414 is used by the controller 412 to provide forautomatic switchover between charging and backup modes. In one example,the “˜MODE” signal 414 is an analogue input to a comparator (e.g., 600of FIG. 6) referenced to a voltage compatible for TTL or some otherlogic level. This allows ˜MODE 414 to be driven from another circuit orfrom a scaled version of the main input power source. In the simplestrealization, a resistive divider may scale the main input voltage to thecomparator input, and may be scaled to less than the minimum inputvoltage, allowing backup in the case of unexpected supply removal. A“V_CAPACITOR” signal 416 is a JFET input (low input current) and a“V_CAPACITOR_COMMON” signal 418 is high impedance when not sampling thesupercapacitor voltage, i.e., when in backup mode. A “˜ENABLE” signal424 is a signal to enable the entire functionality of the device.

A “I_SENSE” signal 420 is a single input allowing a current input as isneeded in current mode control. In this embodiment, the current throughthe inductor 408 is measured at a current sense 422. The current may infact be measured in several places depending on the topology of circuit.The current sense mechanism of the controller 412 accepts bi-directionalcurrent flow assuming current mode control is used. In the embodiment,the circuit is operated at a high frequency allowing the use of a smallinductor. For simple circuit realization, the internal reference voltageof the controller 412 may be less than both Vcc and the maximum voltageof the supercapacitor 410.

The circuit 401 is appropriate for the configuration where the supplyvoltage, Vcc, is greater than or equal to the maximum allowablecapacitor voltage. In an alternative embodiment the Vcc is lower thanthe maximum allowable capacitor voltage. In this situation the topologyof the charge-discharge circuit 401 of FIG. 4 is reversed so that aboost circuit charges the capacitor and a buck circuit produces Vcc fromthe capacitor voltage.

FIG. 5 illustrates a supercapacitor backup power supply circuit inaccordance with a further embodiment of the present invention. Theconfiguration presented in FIG. 5 is appropriate for a supercapacitorbackup power supply with bi-directional power flow where Vcc is greaterthan the maximum supercapacitor voltage. The controller element of thiscircuit includes diodes, power supply switches for the charge-dischargemechanism, current sensing, voltage sensing, and circuits to support theoperation of the dual mode power supply. The controller element may beimplemented within an integrated circuit (referred to as integratedcircuit 502). A supercapacitor 504 and an inductor 506 are external ofthe integrated circuit 502.

The supercapacitor 504 may be same or similar to the supercapacitor 310of FIG. 3 or the supercapacitor 410 of FIG. 4. The inductor 506 may besimilar to the inductor 308 of FIG. 3 or the inductor 408 of FIG. 4.

The circuit of FIG. 5 has resistor networks similar to those of FIG. 4.A resistor network having resistors 530 and 532 is provided between theintegrated circuit 502 and a node 534 that is a connection node of thesupercapacitor 504 and the inductor 506. A resistor network havingresistors 536 and 538 is provided between Vcc and the integrated circuit502.

In FIG. 5, only resistive elements in the feedback paths are shown,which set the DC potentials. The circuit of FIG. 5 includes two feedbackpaths, only one of which is activated, depending on whether thesupercapacitor 504 is being charged (i.e., charging mode), or discharged(i.e., backup mode). Compensation may be achieved by the addition ofcapacitors to these resistors to provide spectral shaping in order toachieve stable operation of the circuit, in both charging and backupmodes. It is understood by a person of ordinary skill in the art thatmore complex feedback mechanisms may be formed, depending on the desiredoperating characteristics of the circuit.

In FIG. 5, the integrated circuit 502 includes a plurality of pins forINDUCTOR signal 510, V_CAPACITOR signal 512, V_CAPACITOR COMMON signal514, ˜ENABLE signal 516, ˜MODE signal 518, VCC signal 520, V_SENSEsignal 522 and GROUND signal 524. The INDUCTOR signal 510, theV_CAPACITOR signal 512, the V_CAPACITOR_COMMON signal 514, the ˜ENABLEsignal 516, the ˜MODE signal 518, and the V_SENSE signal 522 may besimilar to the I_SENSE signal 420, the V_CAPACITOR signal 418, theV_CAPACITOR_COMMON signal 418, the ˜ENABLE signal 424, the ˜MODE signal414, and the V0_SENSE signal in FIG. 4, respectively.

FIG. 6 illustrates an example of a control circuit in accordance with anembodiment of the present invention. The pin-out of the circuit of FIG.6 is similar to the controller of FIG. 5. In FIG. 6, signals associatedwith the integrated circuit 502 other than the ˜ENABLE signal 516 areshown as examples. The circuit of FIG. 6 is a basic current mode controland, for simplicity, compensation (feedback) elements of the controlloops are not shown.

The ˜MODE input 518 is used to define the operating mode of the circuit(charging or backup) and select the source of the voltage erroramplifier (i.e., 602 or 604) into the inner current loop through aswitch 606. A comparator 600 compares the ˜MODE input 518 with a certainvoltage and operates the switch 606. A comparator 608 compares theoutput of the switch 606 and the output of an “ISENSE” circuit 616.

The circuit 616 includes a resistor 617 and a magnitude and level shiftcircuit 618. The circuit 616 measures the current flowing through theinductor connected at the INDUCTOR node 510. In this embodiment, thismeasurement is a high-side measurement, and the sensing element is notreferred to ground. The circuit 616 thus includes a mechanism totransmit the measured value to the ground-referenced comparator 608 inorder to implement current mode control. The magnitude of the currentflow operates the comparator 608. When the inductor current hits athreshold, e.g., its peak current for the current mode, the current modeis activated.

A latch 610 includes “S” node connected to a clock circuit 612, “R” nodeconnected to the output of the comparator 608, and “Q” node connected toa gate drive circuit 614. The gate drive circuit 614 selects the correctswitch operation for the operating mode (charging or backup), includingoperation of the synchronous rectifier. In FIG. 6, the gate drivecircuit 614 drives switches 620, 622 and 624.

The switch 620 is turned on during the supercapacitor-charging mode. Inbackup mode, the switch 620 is turned off so the resistor network withresistors 530 and 532 of FIG. 5 does not bleed off energy in order tomaximize the backup time available. Granted the power bled off may tendto be small, and thus the switch 620 may be eliminated at the expense ofslightly reduced backup time.

The nature of the ISENSE circuitry (616, 618) depends on how the circuitis constructed. A current transformer is the simplest mechanism ifbuilding the circuit using discrete pans. For silicon implementations,techniques to do high-side current measurements are available to ICdesigners.

In the above embodiments, the main power supply has sufficient hold-uptime such that the backup supply (i.e., the supercapacitor 310 of FIG. 3or 410 of FIG. 4) can detect the missing input power and enter thebackup mode from the charging mode.

In a further embodiment, the intrinsic diodes of the MOSFETs may be usedin lieu of synchronous rectification.

In a further embodiment, additional inputs may be provided to set thepeak inductor current for charging and discharging the supercapacitor,and for compensation of the control loop(s).

FIG. 7 illustrates a supercapacitor based backup power supply circuit inaccordance with a further embodiment of the present invention. Thebackup power supply circuit 700 of FIG. 7 is suitable for theconfiguration where the supply voltage, Vcc, is less than or equal tothe maximum allowable capacitor voltage.

The supply circuit 700 includes switches 702 and 704, diodes 705 and706, an inductor 708, and a supercapacitor 710. The switch 702 isconnected in parallel with the diode 705. The switch 704 is connected inparallel with the diode 706. The inductor 708 and the supercapacitor 710may be same or similar to the inductor 308 and the supercapacitor 304 ofFIG. 3, respectively. In the backup power supply circuit 700, the switch702 and the diode 705 are provided between the inductor 708 and thesupercapacitor 710. The inductor 708 is connected to Vcc node.

In charging mode, the switch 704 is a power switch for the boosting andthe switch 702 acts as a synchronous rectifier. In backup mode, theswitch 702 is a power switch 702 is a power switch for the bucking andthe switch 704 acts as a synchronous rectifier.

It will be appreciated by a person of ordinary skill in the art that thetopology of the based backup power supply circuit is not limited tothose of FIGS. 3, 4 and 7 and other topologies can be envisioned.

The present invention has been described with regard to one or moreembodiments. However, it will be apparent to persons skilled in the artthat a number of variations and modifications can be made withoutdeparting from the scope of the invention as defined in the claims.

What is claimed is:
 1. A system for backup power supply, the systemcomprising: a backup power supply circuit comprising: a supercapacitorfor backing up a potential node; an inductor coupled between a firstterminal of the supercapacitor and the potential node, the inductoroperating in a bi-directional mode to charge and discharge thesupercapacitor; a first switch connected in a first parallel circuitwith a first diode, the first parallel circuit coupled between theinductor and the potential node; and a second switch connected in asecond parallel circuit with a second diode, the second parallel circuitcoupled between the inductor and a second terminal of thesupercapacitor, the first switch being used to modulate flow of currentfrom a power source to the supercapacitor via the inductor when thepower source is available at the potential node, and the second switchbeing used to modulate flow of current from the supercapacitor to thepotential node via the inductor when the power source is lost at thepotential node.
 2. The system according to claim 1, wherein the firstdiode is coupled in parallel to the first switch such that the firstdiode is reverse biased when the power source is available at thepotential node and forward biased when the power source is lost at thepotential node.
 3. The system according to claim 1, wherein the firstdiode is coupled in parallel to the first switch such that the firstdiode is forward biased when the power source is available at thepotential node and reverse biased when the power source is lost at thepotential node.
 4. The system according to claim 1, wherein the firstdiode is an intrinsic diode of the Metal-Oxide-SemiconductorField-Effect Transistor (MOSFET).
 5. The system according to claim 1,wherein the first diode is a schottky diode.
 6. The system according toclaim 1, wherein the second diode is coupled in parallel to the secondswitch such that the second diode is reverse biased when the powersource is available at the potential node and forward biased when thepower source is lost at the potential node.
 7. The system according toclaim 1, wherein the second diode is an intrinsic diode of the MOSFET.8. The system according to claim 1, wherein the second diode is aschottky diode.
 9. The system according to claim 1, wherein at least oneof the first switch and the second switch is a MOSFET.
 10. The systemaccording to claim 1, the backup power supply circuit further comprisinga controller for operating the first switch and the second switch. 11.The system according to claim 10, wherein the controller comprises acharging mode for charging the supercapacitor using the first switch,and a backup mode for discharging the supercapacitor using the secondswitch.
 12. The system according to claim 10, wherein the controllercomprises a current monitor for monitoring the inductor current.
 13. Thesystem according to claim 10, wherein the controller comprises a voltagemonitor for monitoring the voltage level at the potential node.
 14. Thesystem according to claim 10, wherein the controller operates the firstswitch and the second switch to form a buck converter with a synchronousrectifier.
 15. The system according to claim 10, wherein the controlleroperates the first switch and the second switch to form a boostconverter with a synchronous rectifier.